For some time, skin-identical lipids such as ceramides and phytosphingosines have been commercially available for use in cosmetic products. Ceramides and phytosphingosines are high-melting, crystalline substances with only low solubility in cosmetic oils, which considerably impair their incorporation and stabilization in end products. Thus, for example, an oil phase that comprises 0.2% phytosphingosine must be heated to more than 90° C. so that the phytosphingosine is completely dissolved. However, a heating temperature of 90° C. represents a problem for many production plants since this temperature cannot be reached. A further problem that may arise is that during the homogenization in the preparation of an oil-in-water emulsion inversion to an unusable, inhomogeneous water-in-oil emulsion takes place when phytosphingosine is in the oil phase. This phenomenon is observed in particular with more hydrophobic O/W emulsifiers, the so-called lipid emulsifiers.
WO-A-00/53568 describes sphingoid based derivatives and the use thereof. In particular, the '568 reference describes salts which have an essentially improved solubility in an aqueous environment and thus improved efficiency in topical applications. However, due to their electrolyte character, such salts represent a particular emulsion loading and have incompatibility with stabilizers of the hydrocolloid type, such as, for example, carbomer or xanthan gum.
WO-A-99/29293 describes compositions that comprise a combination of a free sphingoid base and a ceramide. The compositions are suitable for application to the human skin and have barrier functions, particularly in the case of skin conditions that are burdened with unordered deregulation of cell growth or of differentiation of inflammation or of an infectious stage. For example, oil-in-water emulsions are described herein which comprise defined emulsifiers. These are said to be suitable for forming a lamellar phase (liquid crystalline or gel phase). The lamellar phases are formed at the oil-water interface of an oil-in-water emulsion and comprise directly the free sphingoid base and the ceramide.
WO-A-98/53797 relates to encapsulated, water-insoluble active ingredients having an amphiphilic character with a content of water and at least one surfactant selected from the group of esters of long-chain carboxylic acids with carboxylic acids containing hydroxyl groups, or salts thereof and the esters of long-chain carboxylic acids with polyalcohols. Such active ingredients are used in the preparation of pharmaceutical, agrochemical or cosmetic formulations. Without more specific working examples, the active ingredients mentioned in the text of the '797 reference are also ceramides, lipophilic amides from a saturated and unsaturated aminodiol radical (diphytosphingosines and sphingosines) and saturated aminotriol radical (phytosphingosines), long-chain fatty acid radicals and long-chain alkyl radicals in the aminodiol moiety. The active ingredient is heated together with the surfactant to 80° C. with stirring and mixed at 75° to 80° C. until a completely clear solution is obtained. After cooling to 25° C. (room temperature), the mixture is heated to about 40° C., water is added and intensive stirring is carried out until the system is completely homogenized.
It is an object of the present invention to provide a way of incorporating skin-identical lipids such as ceramides and phytosphingosine into oil-in-water emulsions which solves the above-described problems of the prior art.